The hindlimb has long been the chief system in which the motor sequencing and control functions of the spinal cord have been studied. Recent work has emphasized the mechanical complexity of the musculoskeletal apparatus and the corresponding complexity of spinal circuits for sensory feedback and reflexes; however, there has been no formal approach to relate the two. Furthermore, the need for systematic application of control theory has been underlined by recent clinical work to restore motor control of paralyzed limbs via functional neuromuscular stimulation and other prosthetic acids. This proposal seeks funds to: 1) continue the development of a 2D mathematical model of the musculoskeletal apparatus and make it available as a personal computer program and as a monograph describing the functional anatomical and biomechanical relationships of the cat hindlimb during normal walking. 2) apply linear quadratic regulator theory to make quantitative predictions of the distribution of proprioceptive feedback from muscles and joints under different kinematic conditions and behavioral objectives of the organism. 3) generalize the modeling process by using automatic equation writing techniques to permit users to generate models of arbitrary physical form, including 3D and multi-legged representations, interactively. Data on the natural EMG and kinesiology from chronically instrumented animal will be combined with musculoskeletal morphometry to permit the step cycle to be decomposed into joint torques (by inverse dynamics) and the work of individual muscles (using an experimentally validated model of EMG- to-force-output). Hypothetical sets of proprioceptors with properties akin to those recorded previously from chronically instrumented animals will be built into a model of the sensory feedback available for responding to small perturbations. Linear quadratic controllers will be designed for a range of optimization criteria that weight energy conservation vs. various measures of kinesiological stability. These feedback matrices and the responses of the model system to applied perturbations will be compared wit the responses to electrical stimulation of muscle nerves (to be measured during treadmill walking in chronically instrumented cats) and with data from other investigators regarding responses to mechanical perturbations of standing posture and projections from proprioceptors via spinal cord circuits.